1. Field of the Invention
The present invention relates to a magnetic switch for a starter, and relates in particular to a construction for reducing the size of a magnetic switch.
2. Description of the Related Art
A conventional magnetic switch for a starter may have a construction like that shown in FIGS. 7 and 8, for example. FIG. 7 is an end elevation of a conventional magnetic switch for a starter, and FIG. 8 is a cross-section taken through line VIII--VIII in FIG. 7.
In FIG. 8, an electromagnetic coil 3 wound onto a coil winding frame 2 or a coil bobbin composed of an insulating body and from which lead wires 18 are drawn is housed in a generally cylindrical switch case 9. A thin-walled cylindrical bush 12 is inserted inside the coil winding frame 2 and is secured by fitting into the switch case 9. A cylindrical fixed core 1 composed of a magnetic body having a collar portion 1a on an end portion is disposed inside the bush 12. Furthermore, a movable core 10 is disposed inside the bush 12 facing the fixed core 1, and a return spring 11 which is a coil spring is compressed and disposed between the fixed core 1 and the movable core 10.
The switch case 9 houses the electromagnetic coil 3 as well as the fixed core 1, and also houses the movable core 10 so as to be able to slide freely on the inner circumferential surface of the bush 12. An end portion of the hollow cylindrical portion of the switch case 9 is thin-walled and forms a thin-walled portion 9a for securing a molded cap 8 molded from resin by crimping. A crimped portion 9b is formed on the extreme end of the thin-walled portion 9a by roll crimping the thin-walled portion 9a of the switch case 9 over a stepped portion 8b disposed around the entire circumference of an end portion of the molded cap 8.
A rod-shaped movable contact shaft 4 slidably supported by the fixed core 1 is disposed inside the fixed core 1. Furthermore, a movable contact 5 which is a flat plate-shaped conductor is disposed on the movable contact shaft 4, being held between insulating washers 6 and 13 composed of insulating material and kept electrically insulated from the movable contact shaft 4. A pushing spring 15 which is a coil spring is compressed and disposed between the insulating washer 6 and a collar portion 4a of the movable contact shaft 4. On the other hand, a retaining ring 14 is disposed in close proximity to the insulating washer 6, being secured by insertion into a groove on the movable contact shaft 4. Consequently, the movable contact 5 is slidably held on the collar portion 4a side of the movable contact shaft 4.
In addition, a return spring 16 which is a coil spring is compressed and disposed between the molded cap 8 and the retaining ring 14 at the molded cap 8 end of the movable contact shaft 4 so as to cover the movable contact shaft 4.
Two fixed contacts 7 comprising generally cylindrical fixed contacts 7b for contacting by the movable contact 5, and fixed contact shafts 7a which are contact terminal conductors are disposed in positions on an inside portion of the molded cap 8 facing the movable contact 5.
The fixed contacts 7 are inserted through the molded cap 8 and fastened. Sheet-shaped packing 19 is held between the fixed contacts 7b and the molded cap 8 to prevent moisture from seeping from outside the molded cap 8 around outer circumferential portions of the fixed contact shafts 7a into the inside of the molded cap 8.
As shown in FIG. 7, on the outside of the molded cap 8, the two fixed contacts 7 constitute an M terminal for electrical connection to a starter motor (not shown) and a B terminal for electric connection to a battery (not shown), respectively, and are disposed so as to have a distance L between B-M terminals which is the distance separating the axes of the two fixed contacts 7. Furthermore, threads 7c are formed on the fixed contact shafts 7a of the fixed contacts 7, and washers 17 forming terminal contact seats connected to the starter motor or the battery are disposed on the threads 7c. Furthermore, a partition portion 8a is convexly disposed on the molded cap 8 between the two fixed contacts 7 to increase the insulation between the fixed contacts.
Furthermore, the pushing spring 15 and the return spring 16 keep the two fixed contacts 7b and the movable contact 5 in positions where they are separated from each other.
Next, the operation of the above will be explained.
When power is supplied to the electromagnetic coil 3 by means of the lead wires 18 so that the electromagnetic coil 3 is in an energized state, the movable core 10 is attracted towards the fixed core 1 and moves the movable contact shaft 4 towards the molded cap 8. The fixed contacts are electrically connected to each other by the movable contact 5 contacting the two fixed contacts 7b. Now, when the movable contact contacts the fixed contacts 7b, the pushing spring 15 is compressed and the movable contact shaft 4 and the movable contact 5 are displaced relative to each other, and only the movable contact shaft 4 moves closer to the molded cap 8.
Furthermore, when the electromagnetic coil 3 is no longer energized, the return spring 16 pushes the movable contact away from the fixed contacts 7b by elastic force.
In a conventional magnetic switch for a starter such as this, the joining of the molded cap 8 and the switch case 9 has involved forming the outside diameter of the molded cap 8 smaller than the outside diameter of the thin-walled portion 9a of the switch case 9 around its entire circumference, and roll crimping the molded cap 8 around its entire circumference with the thin-walled portion 9a of the switch case 9 covering the outer circumference of the molded cap 8 to crimp it.
However, when attempts were made to reduce the size of the magnetic switch by making the outside diameter of the switch case 9 smaller, because the thin-walled portion 9a of the switch case 9 covers the outer circumference of the molded cap 8, it is necessary to make the outside diameter of the molded cap 8 smaller as well. For that reason, it is difficult to maintain the conventional distance L between B-M terminals.
That is to say, if one tries to keep the distance L between B-M terminals as large as possible, the walls of molded cap 8 become thin, leading to insufficient strength in the molded cap 8 and poor filling of the mold during the manufacture of the molded cap 8.
On the other hand, FIGS. 9 and 10 show the construction of a magnetic switch in a case where the magnetic switch is reduced in size by shortening the distance L between B-M terminals. Hereafter, parts or portions the same as or similar to those in FIGS. 7 and 8 will be given the same numbering and duplicate explanations will be omitted.
FIGS. 9 and 10 differ from FIGS. 7 and 8 in that the distance L between B-M terminals has been shortened. In this case, the conventional battery lead terminal for mounting on the B terminal and the motor lead terminal connected to the motor for mounting on the M terminal are obstructed by the partition portion 8a of the molded cap 8 and the lead terminals cannot be mounted.
Furthermore, it becomes difficult to maintain the insulating distance c between the fixed contacts 7b and the movable contact shaft 4 in the magnetic switch. In order to maintain sufficient insulating distance c, it is necessary to reduce the contact surface area between the fixed contacts 7b and the movable contact 5, undesirably increasing the current density at the contact portion.